2004 Grand Cherokee Laredo

I have very aggressive tread tires and a 4 inch lift for offroading plus a winch plate and roof rack. I say all this because it adds weight and reduces my mileage obviously. I get about 12/13 miles to the gallon. My DTE said 0 miles to empty and I still made it twenty more miles. I definitely could have made it further but didn't want to push it any more than I had to as I had my kids with me but I'm thinking I must have a few gallon reserve tank.